Tyre condition monitoring systems a reused to to increase the safety and efficiency of the vehicle. There has been a great deal of interest in tyre monitoring in the past and some examples of recent proposals are disclosed in U.S. Pat. No. 4,703,650, U.S. Pat. No. 4,737,761, U.S. Pat. No. 4,823,107, U.S. Pat. No. 4,837,553, U.S. Pat. No. 4,843,872, U.S. Pat. No. 4,893,110, U.S. Pat. No. 5,029,468 and U.S. Pat. No. 5,054,315.
In our International Patent Application No.
PCT/GB 93/02005 published as WO-A-94/06640 we describe a tyre condition monitoring system comprising a unit mountable in a wheel of a vehicle. The unit comprises a sensor, a voltage controlled oscillator and a code generator arranged such that a coded signal is generated in a time period related to the value of the pressure or temperature sensed by the sensor. In order to conserve power the unit comprises a power supply which is activated by a timer from time to time causing the coded signal to be transmitted. Once the code has been transmitted the power supply is deactivated. The unit further comprises a monitor circuit which continuously monitors the sensor for an unacceptable pressure or temperature condition. An override circuit is responsive to the monitor circuit or to an external stimulus to activate the power supply.
International Patent Application No. PCT/CA 92/00072 published as WO-A-92/14620 describes a tyre monitoring apparatus and method in which a code representing a measured physical quantity, property or condition of a tyre is transmitted. The circuit is operable in an active mode in which a measurement circuit measures an instantaneous value of temperature and pressure and a transmitter circuit transmits a signal representing the sensed instantaneous values of pressure and temperature. In the low power mode minimal power is consumed by the measurement and transmitter circuits.